Most television receivers include automatic frequency control (AFC) or automatic fine tuning (AFT) systems for maintaining the fine tuning of the receiver for a selected channel during the operation of the receiver, thereby improving the quality of the received picture. These systems are particularly desirable in color television receivers where relatively slight mistuning can result in a loss of the color portion of the picture or in distortion of the color portion.
A problem exists with AFC or AFT systems, however, during changing of the tuning of the receiver from one channel to another. It is possible for the AFC circuit to lock to a frequency other than the desired frequency for the new channel, and no useable picture or sound will be reproduced. This condition is called "lock-out". Because of this, it is often desirable to disable the AFC circuits during channel change. In the past this has been accomplished by an AFC defeat switch operated by the mechanical motion of the television tuner shaft during the channel change. Other techniques used with remote control receivers have sensed the operation of the remote control tuning motor to defeat or remove the AFC voltage during the channel changing.
The advent of electronic tuning, however, has resulted in a situation where mechanical motion is not necessarily present during a channel change. Electronic tuning systems can use rotary switches similar to those previously used in television receivers for effecting channel change, but this is not necessary. As a consequence, it is desirable to disable the AFC or AFT circuit during channel change in an electronically tuned receiver in response to a condition of the tuning voltage which occurs during such channel change. Then AFC defeat would not be dependent upon mechanical movement for its operation, but would operate directly in response to the condition of the tuning voltage for the receiver.